Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

44 Chapter 2: Cell Chemistry and Bioenergetics
ENERGY
CONTENT
(kJ/mole)
average
thermal motions
ATP
hydrolysis
in cell
C–C bond
breakage
1 10 100 1000 10,000 kJ
noncovalent bond
breakage in water
green
light
complete
glucose oxidation
which are much weaker (Figure 2–2). We shall see later that noncovalent bonds
are important in the many situations where molecules have to associate and dissociate
readily to carry out their biological functions.
Water Is Held Together by Hydrogen Bonds
MBoC6 m2.07/2.02
The reactions inside a cell occur in an aqueous environment. Life on Earth began
in the ocean, and the conditions in that primeval environment put a permanent
stamp on the chemistry of living things. Life therefore hinges on the chemical
properties of water, which are reviewed in Panel 2–2, pp. 92–93.
In each water molecule (H 2 O) the two H atoms are linked to the O atom by
covalent bonds. The two bonds are highly polar because the O is strongly attractive
for electrons, whereas the H is only weakly attractive. Consequently, there is
an unequal distribution of electrons in a water molecule, with a preponderance
of positive charge on the two H atoms and of negative charge on the O. When
a positively charged region of one water molecule (that is, one of its H atoms)
approaches a negatively charged region (that is, the O) of a second water molecule,
the electrical attraction between them can result in a hydrogen bond. These
bonds are much weaker than covalent bonds and are easily broken by the random
thermal motions that reflect the heat energy of the molecules. Thus, each
bond lasts only a short time. But the combined effect of many weak bonds can be
profound. For example, each water molecule can form hydrogen bonds through
its two H atoms to two other water molecules, producing a network in which
hydrogen bonds are being continually broken and formed. It is only because of
the hydrogen bonds that link water molecules together that water is a liquid at
room temperature—with a high boiling point and high surface tension—rather
than a gas.
Molecules, such as alcohols, that contain polar bonds and that can form
hydrogen bonds with water dissolve readily in water. Molecules carrying charges
(ions) likewise interact favorably with water. Such molecules are termed hydrophilic,
meaning that they are water-loving. Many of the molecules in the aqueous
environment of a cell necessarily fall into this category, including sugars, DNA,
RNA, and most proteins. Hydrophobic (water-hating) molecules, by contrast, are
uncharged and form few or no hydrogen bonds, and so do not dissolve in water.
Hydrocarbons are an important example. In these molecules all of the H atoms are
covalently linked to C atoms by a largely nonpolar bond; thus they cannot form
effective hydrogen bonds to other molecules (see Panel 2–1, p. 90). This makes the
hydrocarbon as a whole hydrophobic—a property that is exploited in cells, whose
membranes are constructed from molecules that have long hydrocarbon tails, as
we see in Chapter 10.
Figure 2–2 Some energies important
for cells. A crucial property of any bond—
covalent or noncovalent—is its strength.
Bond strength is measured by the amount
of energy that must be supplied to break
it, expressed in units of either kilojoules
per mole (kJ/mole) or kilocalories per mole
(kcal/mole). Thus if 100 kJ of energy must
be supplied to break 6 × 10 23 bonds of
a specific type (that is, 1 mole of these
bonds), then the strength of that bond is
100 kJ/mole. Note that, in this diagram,
energies are compared on a logarithmic
scale. Typical strengths and lengths of the
main classes of chemical bonds are given
in Table 2–1.
One joule (J) is the amount of energy
required to move an object a distance of
one meter against a force of one Newton.
This measure of energy is derived from the
SI units (Système Internationale d’Unités)
universally employed by physical scientists.
A second unit of energy, often used by
cell biologists, is the kilocalorie (kcal); one
calorie is the amount of energy needed to
raise the temperature of 1 gram of water by
1°C. One kJ is equal to 0.239 kcal (1 kcal
= 4.18 kJ).
Four Types of Noncovalent Attractions Help Bring Molecules
Together in Cells
Much of biology depends on the specific binding of different molecules caused by
three types of noncovalent bonds: electrostatic attractions (ionic bonds), hydrogen
bonds, and van der Waals attractions; and on a fourth factor that can push
molecules together: the hydrophobic force. The properties of the four types of
noncovalent attractions are presented in Panel 2–3 (pp. 94–95). Although each